1. Field of the Invention
This invention concerns improvements in and relating to a process for producing sound ductile cast iron crank shafts with the use of no, or substantially no, blind riser portions at the upper end of the crank shaft portions in the bottom gating-type of vertical pouring system.
In this specification the term "substantially no riser" includes an extension of the casting as the machinery allowance to accommodate a sinking in the casting.
2. Prior Art
As is well known, ductile cast iron is widely employed in various industrial areas because of its excellent physical properties. However, its use is apt to result in various types of casting defects as compared with castings formed from flake graphite cast iron, especially in the formation of shrinkage cavities. Therefore, it is conventional practice to attach large risers or shrinkers to the casting proper and, moreover, to apply an exothermic sleeve onto the riser or to provide a chiller at casting parts exhibiting the shrinkage defects, in order to eliminate such shrinkage defects. As a result of such conventional expedients, the yield of ductile iron casting (casting weight/pouring weight) is much reduced, to as little as 50% or less, and even to 40% or less in some cases.
The reason why ductile cast iron possesses greater shrinkage characteristics as compared with flake cast iron is considered to be that growth of the external solidified layer is slower and that solidification of the poured metal starts, not only from the external surface, but also from the internal nuclei, which are formed during the progress of the metal solidification, and that the solidification expansion of ductile cast iron is considerably large. In other words, since the external solidified layer (shell layer) of ductile cast iron is still thin and weak when the larger volume of internal liquid metal begins to solidify, solidification expansion of the internal liquid metal results, accompanied by enlargement of the casting as well as any internal voids and possibly resulting in the formation of fresh cavities. On the contrary, since the shell layer (external solidified layer) of flake cast iron is thick and strong at the time when the internal liquid metal begins to solidify, isolated eutectic cells can displace liquid metal into any initially-formed shrinkage voids.
Further, since the thickness of various sections of the castings varies widely and the thin sections solidify before the heavy sections solidify, the shrinkage occurs because of insufficient feeding of melt into the shrinking section. This is especially the case with ductile cast iron. Extremely small sized articles or uniform thin-walled articles can be cast soundly with no, or substantially no, risers, but this is the rare case. The riser is generally considered to be essential to the production of medium or large sized sound ductile iron castings. More particularly, the riser is considered to be essential to the production of ductile cast iron crank shafts. Therefore, it is usual practice to attach blind risers at the upper ends of the cast crank shafts larger than those employed with flake cast iron, to provide a chiller at the thick-walled zone to promote uniform cooling throughout the casting and/or to apply an exothermic sleeve to the neck of the riser to increase the feeding effect for the production of sound ductile cast iron crank shafts.
It is known that the formation of shrinkage cavities is reduced by pouring molten metal with low solidification expansion properties into a rigid mold and to cool the molten metal at a uniform rate, but a technique for the production of sound ductile cast iron crank shafts with the use of no, or substantially no, blind risers at the upper ends of the cast crank shafts has yet to be established.
3. Objects of the Invention
Therefore, an object of this invention is to produce sound ductile cast iron crank shafts without chillers and also with the use of no, or substantially no, blind risers at the upper ends of the case crank shafts in the bottom gating-type vertical pouring system, the production of which crank shafts would otherwise require the use of ordinary risers and chillers.
Another object of this invention is to produce ductile cast iron crank shafts in high yield and accordingly with low cost.
Other objects of the invention will be obvious to those skilled in the art as the description of this invention proceeds.